I fitted a cheap Ebay special, a cat back system, £134 delivered from Poland and it is excellent. when I bought my car it had a boy racer exhaust that made the car really uncomfortable to drive after about 15-20 mins. It was like being in the cockpit of a World War 2 fighter.
My car has gone through 2 MOT's with my cheapie exhaust on with no problems what so ever. Although I refer to it as a cheap system it was made in Germany by Thyssen or Thrupp if my memory serves me correctly, so could really be OEM.
I have just bought an exhaust for my Merc 230ce from the same seller and all I paid was £70 delivered, that's £58 for the full exhaust system (no cat fitted) and £12 tracked delivery from Poland.